A hallucinogen is a psychoactive agent which can cause hallucinations, perception anomalies, and other substantial subjective changes in thoughts, emotion, and consciousness.The common types of hallucinogens are psychedelics, dissociatives, or deliriants. By contrast, Stimulants, Opioids, and other psychoactive drugs are not explicitly hallucinogens because a 'hallucination' is visual terminology. The psychoactivity of opioids is devoid of visual anomalies, though the 'numbing' can be considered dissociation from pain. Hallucinations are not an uncommon symptom of amphetamine psychosis, but as they are not a primary effect of the drugs themselves. While stimulants do not induce hallucinations without abuse, the nature of stimulant psychosis is not unlike delirium.

L. E. Hollister's criteria for establishing that a drug is hallucinogenic are as follows:[1]

in proportion to other effects, changes in thought, perception, and mood should predominate;

intellectual or memory impairment should be minimal;

stupor, narcosis, or excessive stimulation should not be an integral effect;

A debate persists on criteria which would easily differentiate a substance which is 'psychedelic' from one 'hallucinogenic'. Sir Thomas Browne in 1646 coined the term 'hallucination' from the Latin word "alucinari" meaning "to wander in the mind". The term 'psychedelic' is derived from the Ancient Greek words psychē (ψυχή, "mind") and dēloun (δηλοῦν, "to make visible, to reveal"), or "mind-revealing".

'A hallucinogen' and 'a psychedelic' may refer correctly to the same substance. 'Hallucinations' and 'psychedelia' may both refer to the same aspects of subjective experience in a given instance. The term psychedelia carries an added reference to psychedelic substance culture, and 'psychedelics' are considered by many to be the 'traditional' or 'classical hallucinogens' including p. cubensis, Mescaline, and LSD. 'A hallucinogen' in this sense broadly refers to any substance which causes perceptual disturbances or hallucinations, while psychedelics carry a positive connotation of general perceptual enhancement. In contrast to Hollister's original criteria, adverse effects may predominate with some hallucinogens with this application of the term.[2]

The word psychedelic (From Ancient Greekψυχή (psychê) mind, soul + δηλος (dêlos) manifest, reveal + -ic) was coined to express the idea of a drug that makes manifest a hidden but real aspect of the mind. It is commonly applied to any drug with perception-altering effects such as LSD and other ergotamine derivatives, DMT and other tryptamines including the alkaloids of p. cubensis, mescaline and other phenethylamines.

The term "psychedelic" is applied somewhat interchangeably with "psychotomimetic" and "hallucinogen",[3] The classical hallucinogens are considered to be the representative psychedelics and LSD is generally considered the prototypical psychedelic.[3] In order to refer to the LSD-like psychedelics, scientific authors have used the term "classical hallucinogen" in the sense defined by Glennon (1999): "The classical hallucinogens are agents that meet Hollister's original definition, but are also agents that: (a) bind at 5-HT2 serotonin receptors, and (b) are recognized by animals trained to discriminate 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM) from vehicle.[4] Otherwise, when the term "psychedelic" is used to refer only to the LSD-like psychedelics (a.k.a. the classical hallucinogens), authors explicitly point that they intend "psychedelic" to be understood according to this more restrictive interpretation (e.g. see Nichols, 2004).[2]

One explanatory model for the experiences provoked by psychedelics is the "reducing valve" concept, first articulated in Aldous Huxley's book The Doors of Perception.[5] In this view, the drugs disable the brain's "filtering" ability to selectively prevent certain perceptions, emotions, memories and thoughts from ever reaching the conscious mind. This effect has been described as mind expanding, or consciousness expanding, for the drug "expands" the realm of experience available to conscious awareness.

It is not known when research chemicals have been first produced, however since the mid-2000s there has been large production of hundreds of virtually unstudied hallucinogens intended for research of nerve signalling pathways and cellular metabolism. Notably the napthalene group on certain cannabimimetrics is considered carcinogenic because it lacks a metabolic path out of the body.[citation needed] Synthetic substances are unrecognised by human physiology and have the potential to form toxic metabolites. The LSD molecule is small enough to enter intracellular space, where it has potent though not deadly effects.[citation needed] There is ample evidence that LSD can alter transcription factors and modulate other DNA processes.[citation needed] Comparatively, 25I-NBOMe (commonly misrepresented as LSD) though of similar duration to LSD is much more acute and users often show signs of explicit toxicity.[citation needed] 25I-NBOMe has been implicated in several deaths.[citation needed] Questions as to the nature of the toxicity of 25I-NBOMe and many other hallucinogens remain pending investigation.

An assumption of safety should not be made for pure forms of traditional psychedelics. Tryptamines rarely have lingering toxicity (see bufotenin) but their effects are more accurately characterized as disrupting some metabolic process. Psilacetin can disrupt acetyl (fat) metabolism, while psilocybin competes for dephosphorylation with other molecules which normally produce energy such as cyclic adenosine monophosphate. This in effect turns the body anabolic for the duration of the conversion of these alkaloids into psilocoin. In a similar way, DMT disrupts pH and water homeostasis and[citation needed], in the traditional preparation of ayahuasca, may assist in DNA methylation. It is possible to overdose on tryptamines.

Tryptamines have agonist action as direct structural homolouges of serotonin. Phenethylamines may contain features which prevent their degradation except through certain routes. 65% of MDMA is excreted unchanged in the urine, and between 20% and 50% of mescaline is excreted unchanged in urine. Mescaline is however subject to some MAO degradation. As more groups are added (substituted) in phenethylamines in the search of a structure which has most binding affinity for 5HT receptor subtypes, the metabolism becomes vastly more complex and the potential for toxic or carcinogenic metabolites to be produced increases greatly.[citation needed]

Dissociatives produce analgesia, amnesia and catalepsy at anesthetic doses.[6] They also produce a sense of detachment from the surrounding environment, hence "the state has been designated as dissociative anesthesia since the patient truly seems disassociated from his environment."[7] Dissociative symptoms include the disruption or compartmentalization of "...the usually integrated functions of consciousness, memory, identity or perception."[8]p. 523 Dissociation of sensory input can cause derealization, the perception of the outside world as being dream-like or unreal. Other dissociative experiences include depersonalization, which includes feeling detached from one's body; feeling unreal; feeling able to observe one's actions but not actively take control; being unable to recognize one's self in the mirror while maintaining rational awareness that the image in the mirror is the same person.[9][10][11] Simeon (2004) offered "...common descriptions of depersonalisation experiences: watching oneself from a distance (similar to watching a movie); candid out-of-body experiences; a sense of just going through the motions; one part of the self acting/participating while the other part is observing;...."[12]

Some dissociatives can have CNSdepressant effects, thereby carrying similar risks as opioids, which can slow breathing or heart rate to levels resulting in death (when using very high doses). DXM in higher doses can increase heart rate and blood pressure and still depress respiration. Inversely, PCP can have more unpredictable effects and has often been classified as a stimulant and a depressant in some texts along with being as a dissociative. While many have reported that they "feel no pain" while under the effects of PCP, DXM and Ketamine, this does not fall under the usual classification of anesthetics in recreational doses (anesthetic doses of DXM may be dangerous). Rather, true to their name, they process pain as a kind of "far away" sensation; pain, although present, becomes a disembodied experience and there is much less emotion associated with it. As for probably the most common dissociative, nitrous oxide, the principal risk seems to be due to oxygen deprivation. Injury from falling is also a danger, as nitrous oxide may cause sudden loss of consciousness, an effect of oxygen deprivation. Because of the high level of physical activity and relative imperviousness to pain induced by PCP, some deaths have been reported due to the release of myoglobin from ruptured muscle cells. High amounts of myoglobin can induce renal shutdown.[17]

Many users of dissociatives have been concerned about the possibility of NMDA antagonist neurotoxicity (NAN). This concern is partly due to William E. White, the author of the DXM FAQ, who claimed that dissociatives definitely cause brain damage.[18] The argument was criticized on the basis of lack of evidence[19] and White retracted his claim.[20] White's claims and the ensuing criticism surrounded original research by John Olney.

In 1989, John Olney discovered that neuronal vacuolation and other cytotoxic changes ("lesions") occurred in brains of rats administered NMDA antagonists, including PCP and ketamine.[21] Repeated doses of NMDA antagonists led to cellular tolerance and hence continuous exposure to NMDA antagonists did not lead to cumulative neurotoxic effects. Antihistamines such as diphenhydramine, barbiturates and even diazepam have been found to prevent NAN.[22]LSD and DOB have also been found to prevent NAN.[23]

Deliriants, as their name implies, induce a state of delirium in the user, characterized by extreme confusion and an inability to control one's actions. They are called deliriants because their subjective effects are similar to the experiences of people with delirious fevers.

In addition to the dangers of being far more distracted from or unable to distinguish reality than with other drugs and retaining a truly fragmented dissociation from regular consciousness without being immobilized, the anticholinergics are toxic, carry the risk of death by overdose, and also include a number of uncomfortable side effects. These side effects usually include dehydration and mydriasis (dilation of the pupils).

Most modern-day psychonauts who use deliriants report similar or identical hallucinations and challenges. For example, diphenhydramine, as well as dimenhydrinate, when taken in a high enough dosage, often are reported to evoke vivid, dark, and entity-like hallucinations, peripheral disturbances, feelings of being alone but simultaneously of being watched, and hallucinations of real things ceasing to exist. Deliriants also may cause confusion or even rage, and thus have been used by ancient peoples as a stimulant before going into battle.[25]

In the context of religious practice, psychedelic drug use, as well as other substances such as tobacco (hypnotic), are referred to as entheogens. In some places peyote is classified as 'sacrament' for part of religious ceremonies, and is legally condoned for such use.

Hallucinogenic substances are among the oldest drugs used by human kind, as hallucinogenic substances naturally occur in mushrooms, cacti and a variety of other plants. Numerous cultures worldwide have endorsed the use of hallucinogens in medicine, religion and recreation, to varying extents, while some cultures have regulated or outright prohibited their use. In most developed countries today, the possession of many hallucinogens, even those found commonly in nature, is considered a crime punishable by fines, imprisonment or even death. In some countries, such as the United States and the Netherlands, partial deference may be granted to traditional religious use by members of indigenous ethnic minorities such as the Native American Church and the Santo Daime Church. Recently the União do Vegetal, a Christian-based religious sect whose composition is not primarily ethnicity-based, won a United States Supreme Court decision authorizing its use of ayahuasca. However, in Brazil, ayahuasca use in a religious context has been legal since 1987. In fact, it is a common belief among members of the União do Vegetal that ayahuasca presents no risk for adolescents within the church, as long as they take it within a religious context.[26]

Shamans consume hallucinogenic substances in order to induce a trance. Once in this trance, shamans are able to communicate with the spirit world, and can often see what is causing their patients illness. The Aguaruna of Peru believe that many illnesses are caused by the darts of sorcerers. Under the influence of yaji, a hallucinogenic drink, Aguaruna shamans are able to discover and remove darts from their patients.[28]

Although natural hallucinogenic drugs have been known to mankind for millennia, it was not until the early 20th century that they received extensive attention from Westernscience. Earlier beginnings include scientific studies of nitrous oxide in the late 18th century, and initial studies of the constituents of the peyote cactus in the late 19th century. Starting in 1927 with Kurt Beringer's Der Meskalinrausch (The Mescaline Intoxication), more intensive effort began to be focused on studies of psychoactive plants. Around the same time, Louis Lewin published his extensive survey of psychoactive plants, Phantastica (1928). Important developments in the years that followed included the re-discovery of Mexicanpsilocybin mushrooms (in 1936 by Robert J. Weitlaner) and Christmas vine (in 1939 by Richard Evans Schultes). Arguably the most important pre-World War II development was by Albert Hofmann's 1938 discovery of the semi-synthetic drug LSD, which was later discovered to produce hallucinogenic effects in 1943.

After World War II there was an explosion of interest in hallucinogenic drugs in psychiatry, owing mainly to the invention of LSD. Interest in the drugs tended to focus on either the potential for psychotherapeutic applications of the drugs (see psychedelic psychotherapy), or on the use of hallucinogens to produce a "controlled psychosis", in order to understand psychotic disorders such as schizophrenia. By 1951, more than 100 articles on LSD had appeared in medical journals, and by 1961, the number had increased to more than 1000 articles.[29] Hallucinogens were also researched in several countries for their potential as agents of chemical warfare. Most famously, several incidents associated with the CIA's MK-ULTRAmind control research project have been the topic of media attention and lawsuits.

At the beginning of the 1950s, the existence of hallucinogenic drugs was virtually unknown to the general public in the West. However this soon changed as several influential figures were introduced to the hallucinogenic experience. Aldous Huxley's 1953 essay The Doors of Perception, describing his experiences with mescaline, and R. Gordon Wasson's 1957 Life magazine article (Seeking the Magic Mushroom) brought the topic into the public limelight. In the early 1960s, counterculture icons such as Jerry Garcia, Timothy Leary, Allen Ginsberg and Ken Kesey advocated the drugs for their psychedelic effects, and a large subculture of psychedelic drug users was spawned. Psychedelic drugs played a major role in catalyzing the vast social changes initiated in the 1960s.[30][31] As a result of the growing popularity of LSD and disdain for the hippies with whom it was heavily associated, LSD was banned in the United States in 1967.[32] This greatly reduced the clinical research about LSD, although limited experiments continued to take place, such as those conducted by Reese Jones in San Francisco.[33]

As early as the 1960s, research into the medicinal properties of LSD was being conducted. It has been found that LSD is a fairly effective treatment for mental disorders such as obsessive compulsive disorder (OCD). "Savage et al. (1962) provided the earliest report of efficacy for a hallucinogen in OCD, where after two doses of LSD, a patient who suffered from depression and violent obsessive sexual thoughts experienced dramatic and permanent improvement (Nichols 2004: 164)." LSD, along with other hallucinogens, possesses a considerable amount of medicinal properties, which is why further research on the medical uses of hallucinogens is paramount.[34]

Yet the most popular, and at the same time most stigmatized, use of psychedelics in Western culture has been associated with the search for direct religious experience, enhanced creativity, personal development, and "mind expansion". The use of psychedelic drugs was a major element of the 1960s counterculture, where it became associated with various social movements and a general atmosphere of rebellion and strife between generations.

Despite prohibition, the recreational, spiritual, and medical use of psychedelics continues today. Organizations, such as MAPS and the Heffter Research Institute, have arisen to foster research into their safety and efficacy, while advocacy groups such as the Center for Cognitive Liberty and Ethics push for their legalization. In addition to this activity by proponents, hallucinogens are also widely used in basic science research to understand the mind and brain. However, ever since hallucinogenic experimentation was discontinued in the late 1960s, research into the therapeutic applications of such drugs have been almost nonexistent, that is until this last decade where research has finally been allowed to resume.

In Canada, mescaline is listed as prohibited under schedule III of the Controlled Drugs and Substances Acts, but peyote is specifically exempt and legally available in Canada.

As of 2008, most well-known hallucinogens (aside from dextromethorphan, diphenhydramine and dimenhydrinate) are illegal in most Western countries. In the United States hallucinogens are classified as a schedule 1 drug. The 3-pronged test for schedule 1 drugs is as follows: the drug has no currently accepted medical use, there is a lack of safety for the use of the drug under medical supervision, and the substance has a high potential for abuse.[39] One notable exception to the current criminalization trend is in parts of Western Europe, especially in the Netherlands, where cannabis is considered to be a "soft drug". Previously included were hallucinogenic mushrooms, but as of October 2007 the Netherlands officials have moved to ban their sale following several widely publicized incidents involving tourists. While the possession of soft drugs is technically illegal, the Dutch government has decided that using law enforcement to combat their use is largely a waste of resources. As a result, public "coffeeshops" in the Netherlands openly sell cannabis for personal use, and "smart shops" sell drugs like Salvia divinorum, and until the ban of psilocybin mushrooms took effect, they were still available for purchase in smartshops as well. (See Drug policy of the Netherlands).

Despite being scheduled as a controlled substance in the mid-1980s, MDMA's popularity has been growing since that time in western Europe and in the United States.

Attitudes towards hallucinogens other than cannabis have been slower to change. Several attempts to change the law on the grounds of freedom of religion have been made. Some of these have been successful, for example the Native American Church in the United States, and Santo Daime in Brazil. Some people argue that a religious setting should not be necessary for the legitimacy of hallucinogenic drug use, and for this reason also criticize the euphemistic use of the term "entheogen". Non-religious reasons for the use of hallucinogens including spiritual, introspective, psychotherapeutic, recreational and even hedonistic motives, each subject to some degree of social disapproval, have all been defended as the legitimate exercising of civil liberties and freedom of thought.

Several medical and scientific experts, including the late Albert Hofmann, advocate the drugs should not be banned, but should be strongly regulated and warn they can be dangerous without proper psychological supervision.[40]

Most psychedelics are not known to have long-term physical toxicity. However, entactogens such as MDMA that release neurotransmitters may stimulate increased formation of free radicals possibly formed from neurotransmitters released from the synaptic vesicle.[citation needed] Free radicals are associated with cell damage in other contexts, and have been suggested to be involved in many types of mental conditions including Parkinson's disease, senility, schizophrenia, and Alzheimer's. Research on this question has not reached a firm conclusion. The same concerns do not apply to psychedelics that do not release neurotransmitters, such as LSD, nor to dissociatives or deliriants.

No clear connection has been made between psychedelic drugs and organic brain damage. However, hallucinogen persisting perception disorder (HPPD) is a diagnosed condition wherein certain visual effects of drugs persist for a long time, sometimes permanently, although science and medicine have yet to determine what causes the condition.

LSD, peyote, psilocybin, and PCP are drugs that cause hallucinations, which can alter a person’s perception of reality. LSD, peyote, and psilocybin cause their effects by initially disrupting the interaction of nerve cells and the neurotransmitter serotonin.[41] It is distributed throughout the brain and spinal cord, where the serotonin system is involved with controlling of the behavioral, perceptual, and regulatory systems. This also includes mood, hunger, body temperature, sexual behavior, muscle control, and sensory perception. Certain hallucinogens, such as PCP act through a glutamate receptor in the brain which is important for perception of pain, responses to the environment, and learning and memory. Thus far, there have been no properly controlled research studies on the specific effects of these drugs on the human brain, but smaller studies have shown some of the documented effects associated with the use of hallucinogens.[42]

The class of drugs described in this article has been described by a profusion of names, most of which are associated with a particular theory of their nature.

Louis Lewin started out in 1928 by using the word phantastica as the title of his ground-breaking monograph about plants that, in his words, "bring about evident cerebral excitation in the form of hallucinations, illusions and visions [...] followed by unconsciousness or other symptoms of altered cerebral functioning". But no sooner had the term been invented, or Lewin complained that the word "does not cover all that I should wish it to convey", and indeed with the proliferation of research following the discovery of LSD came numerous attempts to improve on it, such as hallucinogen, phanerothyme, psychedelic, psychotomimetic, psychogenic, schizophrenogenic, cataleptogenic, mysticomimetic, psychodysleptic, and entheogenic.

The word psychotomimetic, meaning "mimicking psychosis", reflects the hypothesis of early researchers that the effects of psychedelic drugs are similar to naturally-occurring symptoms of schizophrenia, though it has since been discovered that some psychedelics resemble endogenous psychoses better than others. PCP and ketamine are known to better resemble endogenous psychoses because they reproduce both positive and negative symptoms of psychoses, while psilocybin and related hallucinogens typically produce effects resembling only the positive symptoms of schizophrenia.[43] While the serotonergic psychedelics (LSD, psilocybin, mescaline, etc.) do produce subjective effects distinct from NMDA antagonist dissociatives (PCP, ketamine, dextrorphan), there is obvious overlap in the mental processes that these drugs affect and research has discovered that there is overlap in the mechanisms by which both types of psychedelics mimic psychotic symptoms.[44][45][46] One double-blind study examining the differences between DMT and ketamine hypothesized that classically psychedelic drugs most resemble paranoid schizophrenia while dissociative drugs best mimicked catatonic subtypes or otherwise undifferentiated schizophrenia.[47] The researchers expressed the view that "a heterogeneous disorder like schizophrenia is unlikely to be modeled accurately by a single pharmacological agent."

The word psychedelic was coined by Humphrey Osmond and has the rather mysterious but at least somewhat value-neutral meaning of "mind manifesting". The word entheogen, on the other hand, which is often used to describe the religious and ritual use of psychedelic drugs in anthropological studies, is associated with the idea that it could be relevant to religion. The words entactogen, empathogen, dissociative and deliriant, at last, have all been coined to refer to classes of drugs similar to the classical psychedelics that seemed deserving of a name of their own.

Many different names have been proposed over the years for this drug class. The famous German toxicologist Louis Lewin used the name phantastica earlier in this century, and as we shall see later, such a descriptor is not so farfetched. The most popular names—hallucinogen, psychotomimetic, and psychedelic ("mind manifesting")—have often been used interchangeably. Hallucinogen is now, however, the most common designation in the scientific literature, although it is an inaccurate descriptor of the actual effects of these drugs. In the lay press, the term psychedelic is still the most popular and has held sway for nearly four decades. Most recently, there has been a movement in nonscientific circles to recognize the ability of these substances to provoke mystical experiences and evoke feelings of spiritual significance. Thus, the term entheogen, derived from the Greek word entheos, which means "god within", was introduced by Ruck et al. and has seen increasing use. This term suggests that these substances reveal or allow a connection to the "divine within". Although it seems unlikely that this name will ever be accepted in formal scientific circles, its use has dramatically increased in the popular media and on internet sites. Indeed, in much of the counterculture that uses these substances, entheogen has replaced psychedelic as the name of choice and we may expect to see this trend continue.[2]

Hallucinogens can be classified by their subjective effects, mechanisms of action, and chemical structure. These classifications often correlate to some extent. In this article, they are classified as psychedelics, dissociatives, and deliriants, preferably entirely to the exclusion of the inaccurate word hallucinogen, but the reader is well advised to consider that this particular classification is not universally accepted. The taxonomy used here attempts to blend these three approaches in order to provide as clear and accessible an overview as possible.

Almost all hallucinogens contain nitrogen and are therefore classified as alkaloids. THC and salvinorin A are exceptions. Many hallucinogens have chemical structures similar to those of human neurotransmitters, such as serotonin, and temporarily modify the action of neurotransmitters and/or receptor sites.

A classical classification, mainly of historical interest, is that of Lewin (Phantastica, 1928):

Class I Phantastica roughly correspond to the psychedelics, which is a more modern term usually used as synonym to "hallucinogen" by people with positive attitudes towards them. Here the term is used a bit differently to discriminate one particular class of hallucinogens which it seems to describe best. They typically have no sedative effects (sometimes the opposite) and there is usually a clearcut memory to their effects. These drugs have also been referred to as the "classical" hallucinogens.

Class II Phantastica correspond to the other classes in our scheme. They tend to sedate in addition to their hallucinogenic properties and there often is an impaired memory trace after the effects wear off.

Problems with structure-based frameworks is that the same structural motif can include a wide variety of drugs which have substantially different effects. For example, both methamphetamine and MDMA are substituted amphetamines, but methamphetamine has a much stronger stimulant action than MDMA, with none of the latter's empathogenic effects. Also, drugs commonly act on more than one receptor; DXM, for instance, is primarily dissociative in high doses, but also acts as a serotonin reuptake inhibitor, similar to many phenethylamines and in fact, the phenethylamine moiety is embedded in the structure of DXM. LSD also contains both the indole backbone and the phenethylamine moiety.[citation needed]

Even so, in many cases structure-based frameworks are still very useful, and the identification of a biologically active pharmacophore and synthesis of analogues of known active substances remains an integral part of modern medicinal chemistry.[citation needed]

^Brink Lindsey. The Age of Abundance; How Prosperity Transformed America's Politics and Culture. New York: Collins, 2007. p.156: "...pot and psychedelics revealed to their users wildly different visions of reality from the "straight" one everybody took for granted. ... Guided into those transcendent realms, many young andimpressionable minds were set aflame with visions of radical change. ... Antiwar protesters, feminists, student rebels, environmentalists, and gays all took their turns marching to the solemn strains of "We Shall Overcome"..."

^Goffman, ibidem, p.266–7: "By normative social standards, something unseemly was going on, but since LSD, the catalyst that was unleashing the celebratory chaos, was still legal [in 1966], there was little [the authorities] could do... [That year, a]cross the nation, states started passing laws prohibiting LSD. .... By their panic, as expressed through their prohibitionary legislation, the conservative forces teased out what was perhaps the central countercultural progression for this epoch."

Erowid is a web site dedicated entirely to providing information about psychoactive drugs, with an impressive collection of trip reports, materials collected from the web and usenet, and a bibliography of scientific literature